In medical diagnostics, increasing use is made of imaging methods based on an interaction between an external energy field and the human body. Such a method is magnetic resonance tomography (MRT), also known as nuclear spin tomography. This method uses different examination methods to produce a more precise image of the individual anatomy of the patent and possible anomalies. Such visualizations can be used not only to produce an exact diagnosis, but because of their precision also to simulate and plan operations.
In MRT raw image data capture is carried out by means of high-frequency excitation of nuclear spin of the tissue to be examined in a high basic magnetic field. The high-frequency pulses cause the nuclear spins to be deflected from their position of equilibrium parallel to the basic magnetic field. The nuclear spins then precess around the direction of the basic magnetic field. The magnetic resonance signals thus generated are captured by high-frequency antennas. By appropriately selecting the resonance frequency, it is principally the hydrogen atom nuclei naturally present in large numbers in the human body which are excited. The nuclear spins then gradually relax to their initial position parallel to the basic magnetic field, whereby the magnetic resonance signal to be received correspondingly decreases again. The relaxation times, referred to here as magnetic resonance relaxation times, are among other things material-dependent. A distinction is made here between the T2 relaxation time of the cross-magnetization (transverse to the basic magnetic field) and the T1 relaxation time of the direct-axis magnetization (perpendicular to the cross-magnetization). Cross-sectional images of the section of the patient's body examined are then created from this raw image data. The various capture methods have undergone rapid technical development since their introduction. Image quality has been significantly improved and capture time has been greatly reduced. Precise diagnostic images with a very high level of detail form the basis for an examination of all types of individual organs, such as lungs, liver, stomach, etc., or regions of the body, such as the thorax, head or individual limbs. Furthermore, even catheter examinations in the region of the heart can to an extent be replaced by this technique.
In the case of magnetic resonance tomography, water-cooled components are generally used. The cooling water for instance dissipates the heat which occurs in the gradient coils and high-frequency transmission coils as a result of the high currents. Disadvantageously the high-frequency measurement pulses also mean that the water molecules of the cooling water present in the magnetic resonance measuring device are also excited. These then likewise correspondingly transmit imaging resonance signals. In this case in particular interference is caused in that the cooling water constantly continues to move during an MR measurement, i.e. even during transmission of a high-frequency sequence and up until the readout phase. This can result in significant artifacts in the images captured.
To avoid the disruptive influence of the cooling water on imaging as much as possible, high-frequency (HF) shielding devices have been used to date, with which the water-conducting components are shielded from the measuring space.
In the case of smaller mobile components which are not permanently built in to the magnetic resonance measuring device, such as body coils or head coils designed for measuring on individual parts of the body such as extremities or the head, such shielding is however difficult, in particular if the apparatus is adapted to the body parts to be measured and hence must have a certain degree of flexibility.